Is there a threshold to get these kinds of questions? I've never had to explain where I got a ratty old $20 bill from. I'm also not trying to replace $4,000 in old bills either.

The simplest way to exchange old notes for new is to use them to pay for things while they remain legal tender, which also neatly avoids such questions.

Now, if your government is of the kind that can realistically announce over the weekend that cash is going to be worthless by Monday unless exchanged, then yeah. Then again, if you live in a place like that, you probably already know to keep your money in foreign currency and use the black market exchanges as needed.